This invention relates to a control system for hydraulically driven crane winches of the type having a drum provided with a hydraulically actuated clutch and brake. More specifically the invention pertains to such a control system for selectively actuating the clutch and brake of the drum in order to condition the winch for a selected one of several operating modes.
The hydraulic crane winch system has been known and used extensively wherein one or more winch drums are driven by a combination of hydraulic pump and motor. Each winch drum is normally provided with a hydraulic clutch for connection and disconnection of the drum to and from the hydraulic motor, and a brake for arresting or retarding the rotation of the drum. For the actuation of the clutch it has been customary to employ an operator-controlled valve, separate from a motor control valve, directing hydraulic fluid under pressure into the clutch cylinder for the engagement of the clutch. The mechanism for the actuation of the brake, on the other hand, has been either "positive" or "negative".
By the positive braking mechanism is meant the one whereby the operator can himself apply the brake as by the depression of a brake pedal. Such manual braking offers an advantage as when the drum clutch is disengaged to allow the free fall of the load or the free running-out of the load cable, because the operator can make proper braking control of the drum to stop the load or its suspension device in an exact position desired. The positive braking mechanism presents a hazard, however, as the operator may fail to apply the brake when, with the clutch disengaged, the winch drum is to be brake-locked against rotation.
The noted disadvantage of the positive braking mechanism is absent from the negative braking mechanism, which automatically applies the brake upon disengagement of the drum clutch or upon setting of the drum drive motor in neutral. The negative braking mechanism has its own drawback, however. It includes a brake cylinder of the single-acting, spring-return type, normally holding the brake applied by the force of the spring and releasing the brake upon reception of hydraulic fluid under pressure from the pump of the drum drive system. Consequently, when the drum drive motor is not rotating, the brake is held applied to lock the drum against rotation. Upon disengagement of the clutch for free fall the brake must be released by introducing pressurized fluid into the brake cylinder against the effect of the return spring.
Heretofore, for thus releasing the brake, the brake cylinder has been set out of communication with the circuit of the drum drive motor, and the desired pressurized fluid for actuation of the brake cylinder has been obtained from a pedal-operated master cylinder. Such braking control by hydraulic fluid inevitably incurs time lag, making it difficult to arrest free fall in a desired position. The brake release by depression of the pedal coupled to the master cylinder is also objectionable because it runs counter to the usual braking action by pedaling and so may lead to erroneous operation.
A further problem with the conventional winch control system arises from the presence of too many controls to be manipulated, such as the lever for setting the drum drive motor into and out of forward or reverse rotation, the lever for the on-off control of the drum clutch, and the pedal for the application or release of the drum brake. The operator must actuate these controls in various combinations to obtain a desired mode of operation, so that he must be skilled enough to avoid errors in operation. For example, if the control system includes a positive braking mechanism, the brake pedal should be depressed immediately upon disengagement of the clutch for free fall; otherwise, the load or suspension device would fall too rapidly. Such too rapid fall will also take place with a control system incorporating a negative braking mechanism, if the operator makes an error in the sequence of actuating the brake pedal and clutch lever. He may also erroneously operate the motor control lever when the clutch lever is on, giving rise to unexpected movement of the load or suspension device.